Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a non-permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). It is further known to combine a plurality of such fuel cells into a manifolded structure referred to in the art as a “fuel cell stack” and to provide a partially-oxidized “reformate” fuel (“syngas”) to the stack from a hydrocarbon catalytic reformer.
Prior art catalytic partial-oxidizing (POX) reformers typically are operated exothermically by using intake air to partially oxidize hydrocarbon fuel as may be represented by the following equation for a hydrocarbon and air,C7H12+3.5(O2+3.77N2)→6H2+7 CO+13.22N2+heat  (Eq. 1)wherein the oxygen/carbon atomic ratio is 1.0, and the resulting reformate temperature is in the range of about 1000° C. Prior art reformers typically are operated slightly fuel-lean of stoichiometric to prevent coking of the anodes from non-reformed hydrocarbon decomposition within the fuel cell stack. Thus conventional combustion of hydrocarbon and reformate occurs within the reformer and within the stack in addition to, and in competition with, the electrochemical combustion of the fuel cell process. Such conventional combustion is wasteful of fuel and creates additional heat which must be removed from the reformate and/or stack, typically by passing a superabundance of cooling air through the cathode side of the stack.
It is known to produce a reformate containing hydrogen and carbon monoxide by endothermic steam reforming (SR) of hydrocarbon in the presence of water in the so-called “water gas” process, which may be represented by the following equation,C7H12+7H2O+heat→13H2+7CO  (Eq. 2)wherein the oxygen/carbon atomic ratio is still 1.0 and the reformate temperature is still about 1000° C. The disadvantages of this process for providing reformate for operating a fuel cell are 1) a continuous water supply must be provided; 2) heat must be provided, typically in the form of burned fuel, thus reducing the efficiency of the system; and 3) the reforming temperature is hard on the reformer.
High temperature fuel cells inherently produce a combination of direct current electricity, waste heat, and syngas. The syngas, as used herein, is a mixture of unburned reformate, including hydrogen, carbon monoxide, and nitrogen, as well as combustion products such as carbon dioxide and water. In some prior art fuel cell systems, the syngas is burned in an afterburner, and the heat of combustion is partially recovered by heat exchange to the reformer, to the cathode inlet air, or to both.
In accordance with the invention disclosed in the co-pending and commonly owned patent application 10/793,302, entitled “Apparatus and Method for Operation of a High Temperature Fuel Cell System Using Recycled Anode Exhaust”, a relatively small percentage, typically between 5% and 30%, of the anode syngas may be recycled into the reformer a) to increase fuel efficiency by endothermic reforming of water and carbon dioxide in the syngas in accordance with Equation 2 above (thus combining POX and SR reforming); b) to add excess water to the reformate to increase protection against anode coking; and c) to provide another opportunity for anode consumption of residual hydrogen. In such systems, and especially when using heavy fuels such as gasoline and diesel, the reformer typically is operated at a high temperature (which may even exceed the stack temperature) to provide the energy necessary for endothermic reforming. However, such high temperatures may be deleterious to the reformer over a period of time, and tend to lower system efficiency. From a durability point of view, it is desirable to be able to operate a reformer at the lowest temperature possible (without being in an operating region of carbon formation).
In a fuel cell stack, the reformate consumed is converted into approximately equal amounts of heat and electricity. The stack is cooled primarily by the flow of gases through it. Even with a modest amount of recycle flow added to the reformate, the total reformate massflow is relatively small, on the order of one-tenth the massflow of the cathode air, so the majority of cooling is done by cathode air. As previously noted, in endothermic reforming of recycled syngas, the reformate produced cannot be cooled below stack temperature without risk of carbon nucleation. Therefore, in order to keep a reasonable temperature gradient in the stack between the inlet and outlet of the cathode, a very high cathode air massflow is required, being many times the amount required for the electrochemistry of the stack. This creates an added energy parasitic to the stack in the form of a very large air blower, and also tends to make the size of the cathode heat exchanger much larger than would otherwise be necessary.
Gas turbines are well known for use in driving electric generators. Typically, a stream of hot gas is impinged against one or more bladed turbines mounted on a shaft of the generator. A typical gas turbine generator operates at about 38% fuel efficiency.
It is well known that a solid oxide fuel cell and a gas turbine can be integrated into a combined system known in the art as an SOFC/GT hybrid. Such systems have been commercialized on a limited or experimental basis, for example, by Siemens Westinghouse Power Corporation, using all ceramic high temperature stacks. Such a fuel cell system, running at a temperature of approximately 1000° C., is well matched to the thermal requirements of uncooled gas turbines, the system is inherently large and heavy; is high in materials and processing cost; and is slow in start-up time and in transient response.
Intermediate temperature SOFC stacks, operating at a nominal stack temperature between about 700° C. and 850° C. are expected to be smaller, less expensive, and more capable of fast start-up and transient response. However, the operating temperature is not so well suited to gas turbine integration.
As noted above, conventional cooling strategies for a conventional SOFC require considerable excess air on the cathode side and modest temperature differentials between the inlet and the exit of the stack. This results in a need for a relatively large heat exchanger for cathode air preheating, and a relatively large blower or compressor to supply air to the fuel cell.
What is needed in the art is a new configuration for an SOFC/GT hybrid that optimizes thermodynamic coupling of the SOFC and GT processes and allows the hybrid system to be operated with substantially less heat exchange required in fuel vaporization/reforming and in cathode air preheating. Such an optimized hybrid offers large reductions in system weight and cost, making such an auxiliary power unit (APU) system useful for applications in aerospace, aviation, road/rail/marine transport, and “containerized” distributed power generation.
What is further needed is a means for improving the efficiency of reformer and stack processes while operating the reformer at a temperature below the stack temperature; for minimizing the size and weight of the heat exchangers; and for retaining most or all of the latent heat value of the anode tail gas for downstream processes.
What is still further needed is means for increasing the fuel efficiency of a gas turbine generator.
What is still further needed is auxiliary power unit means for generating electricity which is relatively lightweight and highly fuel efficient.
It is a principal object of the present invention to provide electric power by combining high efficiency operation of a high temperature fuel cell system with a gas turbine generator.